Potential comparator



June 14, 1949. TY ON 2,473,457

POTENTIAL COMPARATOR Filed Aug. 1, 1945 FIG.!

=- fise Ply 193E M'f'fi FFGZ FIG.3

INVENTOR OWEN A. TYSON BY ATTORNlY Patented June 14, 1949 UNITED STATES PATENT OFF ICE mesne assignments,

to the United .States of America as represented by the Secretary of War ApplicationAugust l, 1945, SeriaLNo. 608,314

v 1 Claim. 1 This invention relates to electrical circuits-and more particularly to circuits for producing al- .ternating voltagev from a direct-.currentsignal.

In many velectronic. applications it .is desirable to produce: an alternating-current signal that v-is proportional to the amplitiude of a direct-current signal. .Many times this. is done to avoid having to amplify .the direct-current signal since it .ismoreieasible toamplify alternating current than direct current. It is also desirable in some applications torproduce a signal proportional to thedifierence between two direct- .currentpotentials. L'I'his type of circuitis usurallyiknownas .a D. -'C. comparator circuit.

.. It is an object vof Ithe. present invention,ithere- ,fore; to provide acircuit for producing an al- .ternating-current signal proportional to the difference of two direct-current potentials.

' A further object of thisinventionis'to provide acircuit for producin an alternating-cur- ;rentsignal asa function of the polarity and amplitude lof/a direct-current Signal.

In accordance with the present "invention there is .providedaplurality of amplifiers with .means for applying a plurality of signals'to fat least two .of said amplifiers. Means are'provided for comparing the outputs of at'leasttwoof wsaid. amplifiers in such a manner that'a signal is obtained which isproportional to"therelative amplitudes of two of the input signals.

For-a better understanding of the'lnvention, together with other and further objects thereof, reference :is had to the following description taken in connection with the accompanying drawing-in which:

Fig. 1 is a schematic diagram of -the invention;

Fig. 2 isa plot of the output signals of Fig. 1 as the input signals are varied; and

Fig. 3 is a second plot of the output-signals {with a differentvariation in the'input signals.

1 'This invention employs two identical -triode "vacuum'tubes l-ll--and |2 "-as comparator tubes. Tubes HI "and 12 are connected in identical circuits. The anode M of tube illis connected zto "a suitable source of plate potential through. a

variablere'sistor i6. Theazcatho'de l8 ofirtubeflll is connected to a terminalillionlthe primaryazof a transformer 22. The control grid 24 of tube I is connected to a signal input 26 through a low-pass filter composed of resistor 28 and capacitor 30. A capacitor 32 is connected from anode M to ground.

In a similar manner the anode 40 of tube I2 is connected to a suitable source of plate potential through a variable resistor 42. Anode Mlasalso connected to ground-Joya capacitor-43. The-control grid 44 of .tube l 2:215 connected -.to a signal input 45 through a low-pass .filterecomposed :of .resistor -46 and capacitor 48. Cathode 1500f tube I2 is connected to'a secondrterminal 152cm the primary of transformer :22. A center tap 53' on the primary of transformer 22 is corrnected'to ground through a variable resistorufl. Resistor -54 ispart ofa cathode .fol-lower :circuit that also includes vacuum tube .56. The controlrgrid58 of tube-fifiisrconnected to-ground through resistor 160. A evariable .tapa621on :re- L-sistor "6D is connected "to :signal .-input 6.4. The two terminals 66 and 68 'on the :secondaryof transformer 22 serve as output terminalszfor this circuit.

:If this circuit is to belusedras all-C. voltage comparator, the:two signals to be-compardzane applied tat; signal inputs v 26 and :45. A constant "amplitude sine wave signal is applied to .the grid 58 of the *cathode follower circuit 1 including tube 56. A-sine wave esignalof "somewhat reduced amplitude will appear at the cathode-oi :tube .56. This signal is v-zapplied to "the cathode -18 "of tube .1-0 through one-half of the ,primary of transformerlz. A-sine wavesignal of equal amplitude is applied to cathode '51), tube 12, "through the other half a of r the; primary-f trans- :former'. 22.

30 ;If --thexcontrol grids 24 and. 44- of tube I 0 and l2 rare at v the: same: potential, the. transconductz flfl08 10f thetwo tubes will .be equal {since the ("two tubes have identical characteristics. Since .iequalzsigna-ls -1-are applied-to the-two tubes, the

alternating components of .the plate current' in :two tubes .are aequal. The uplate currents from tubes 10 and I2 flow in opposite vdirections through the primary of transformer. 22, .,1-,herefore, i-no voltage will be induced .in the secondsary of this transformer when wthealternati-ng components :of the two plate currents are-equal. The. conclusion. is, therefore, rthat when the.po- ..tentials applied at signalinputs 2li and AE-ar e .egual, no signal will .appear .at terminals 6 and 68.

-Suppose now-that the potential at signal in- 5.131113: 26 r .is at .a; higher; potential than .the potenttial aaatv signal :input 45. The .cQntroLgr-id .ZILof tube ID will now be at a higher potential than the control grid 44 of tube I2 and the dynamic transconductance of tube ID will be greater than the dynamic transconductance of tube l2. With equal sine wave signals applied to the cathode of each tube the alternatin component of the; plate current through tube ill will be greater',{

than the alternating component of the plate current through tube 12. These unequal components of current will cause an alternating magnetic fiux to be set up in the primary of the transformer 22 and a sine wave signal will appear at terminals 66 and 68 on the secondary of transformer 22. This signal i illustrated as curve of Figs. 2 and 3.

If the potential of signal input 26 is made still higher than that of signal input 45, the transconductance of tube It! will still further increase, and the amplitude of the signal at points 66 and 68 will increase as shown by curve 12 of Fig. 2.

If, however, signal input 45 is made hi her in potential than signal input 26, the transom-- ductance of tube 12 under these conditions is greater than the transconductance of tube ID. The amplitude of the alternating component of plate current through the portion of the primary of transformer 22 between terminals 52 and center tap 53 is greater than the alternating component of plate current through the portion of the primary between terminal and center tap 53. The phase of the voltage induced in the secondary of transformer 22 is the exact opposite of the phase of the voltage induced when signal input 26 is at a higher potential than signal input 45. Therefore the phase of the alternating voltage across the output terminals 66 and 68 with respect to the inin potential between the two signal inputs 26 and 45.

' While it has been stated that tubes In and I2 have identical characteristics, it is recognized 'that it is difficult to select two tubes with exactly identical characteristics so the variable resistors l6 and 42 are provided to compensate for minor differences and characteristics in tubes In and I2. The values of these resistors do not affect the alternating component of plate current through tubes l0 and [2 since the anodes of tubes In and I2 are maintained at a fixed potential for alternating current by capacitors 32 and 40.

The signals that are applied at signal inputs 26 and 45 are supposedly pure direct current signals but it is possible that these signals may have an alternating-current ripple superimposed upon them due to pick-up or some other similar reason.

This ripple would cause inaccurate operation of the circuit so signal inputs 26 and 45 are connected to grids 24 and 44 respectively through .low pass filter circuits that are designed to prevent any alternating current signals from reaching grids 24 and 44. If this circuit is to be used to produce an alternating current voltage proportional to a direct current potential, one of the signal inputs, for example, signal input 45, is

maintained at a fixed potential and the direct current signal is applied at point 26. With a constant amplitude sine wave signal applied at grid 58 of tube 56 the signal at terminals 66 and 68 will be a sine wave voltage whose amplitude is proportional to the amplitude of the signal applied at signal input 26.

The advantages of this circuit as have been pointed out in the above description are that this circuit may be used to compare the amplitudes of two direct-current potentials. Also the circuit may be used to produce an alternating voltage proportional to a direct voltage, thus eliminating the necessity of providing direct-current amplifiers in many applications.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention.

What is claimed is:

A circuit for comparing the amplitudes of two direct-current potentials comprising a first, a second and a third vacuum tube, each having at least a cathode, an anode and a control grid, means for connecting said first vacuum tube as a cathode follower, first and second variable resistors for connecting said anodes of said second and third vacuum tubes to a suitable source of plate potential, said first and second variable resistors providing means for balancing said circuit, first and second capacitor means for maintaining said anodes of said second and third vacuum tubes at a substantially constant potential for alternating voltage signals, a transformer having a first, a second and a third primary terminal and a first and a second secondary terminal, said two secondary terminals serving as the output terminals for said comparator circuit, means for connecting said cathodes of said first, second and third vacuum tubes to said first, second and third primary terminals of said trans former, means for introducing a first and second direct-current potential, low-pass filter means for applying said first and second direct-current potentials to the control grids of said second and third vacuum tubes respectively, and means for applying a sine wave signal to the control grid of said first vacuum tube.

OWEN A. TYSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,586,533 Peterson June 1, 1926 2,129,880 Scherbatskoy Sept. 13, 1938 2,140,662 Zuschlag Dec. 20, 1938 2,179,265 Luck Nov. '7, 1939 2,183,399 Heising Dec. 12, 1939 2,366,603 Dehmel Jan. 2, 1945 2,391,532 Wilmotte Dec. 25, 1945 2,397,540 Dome Apr. 2, 1946 

